1. Field of the Invention
The present invention relates to a process for smoothing a dynamic range of luminance of an image signal, and a color conversion process of an image signal.
2. Description of the Related Art
Some types of digital cameras have a color conversion function that can arbitrarily replace a color of an object image selected by a user with an intended color at the time of shooting. The color conversion function includes a one-point color function or a spot color function. For example, Japanese Patent Application Laid-Open No. 2005-210208 discusses a technique that reads out specified color information from a color database when a replacement target color and a color to be converted are specified by a user. Based on the read color information, color of the digital image data is converted.
According to the technique discussed in Japanese Patent Application Laid-Open No. 2005-210208, the color conversion can be performed when the digital camera performs the shooting. That is, the user can instruct the shooting start while checking a color-converted image using an electronic viewfinder (EVF). Accordingly, it is possible to capture the image that is color-converted according to the user's preference without the inconvenience of importing the image into a personal computer (PC) or the like after the shooting, and editing the imported image.
In a shooting operation using digital cameras, depending on scenes to be shot, there is a big difference in luminance (density) of the images. That is, dynamic ranges of the luminance of the images become very wide, for example, in a case of flash shooting and shooting in backlight. In a case of shooting the scenes that have wide dynamic ranges, in order to smooth the dynamic ranges of film images, a dodging process is performed. In the dodging process, while normal exposure is performed on middle density parts, the light exposure for bright parts is reduced where luminance of images is high and there is a possibility that the images are saturated. On the other hand, the light exposure in dark parts is increased where there is a possibility that the images are deteriorated. By employing the dodging process, the big luminance difference of an image taken on a film is corrected and it is possible to obtain the image that has appropriate luminance over the image such that impression of the image is close to the scene that the user actually saw.
To obtain effects similar to the dodging process for digital images, various techniques have been proposed. For example, Japanese Patent Application Laid-Open No. 2001-257902 discusses a technique that in smoothing a dynamic range of a digital image, prevents giving too sharp impression to a color of the image when the gradation is hardened, or prevents the color of the image from becoming dull when the gradation is softened. In Japanese Patent Application Laid-Open No. 2001-257902, according to a degree of the gradation conversion rate, a multiplying coefficient for each color component is calculated using arithmetic expressions. Then, while the gradation of the image data is being converted, saturation of the color is also converted using the obtained coefficient to realize a further natural image.
However, according to the Japanese Patent Application Laid-Open No. 2005-210208, in a case of a scene having a very wide dynamic range, if a composition of an object is changed while the object image is displayed on an EVF, luminance of the color conversion target changes. Accordingly, as to the same object image, color conversion varies. Moreover, the technique of Japanese Patent Application Laid-Open No. 2001-257902 is directed to adjusting the dynamic range and performing the color conversion process based on the gradation conversion rate. However, the technique discussed in Japanese Patent Application Laid-Open No. 2001-257902 is not directed to converting the color of the object image to the one specified by the user at the time of shooting, that is, the one point color function or the spot color function. Accordingly, when the scene having the wide dynamic range is shot, even if the same object image is captured, the color conversion varies and it is not possible to solve the problem that occurs in Japanese Patent Application Laid-Open No. 2005-210208.